A fuel cell generates electricity directly from a fuel source and an oxidant. The fuel source may be for example, alcohol, hydrogen gas, natural gas, or metal sheet, and the oxidant may be for example, oxygen or air. Because this process does not "burn" the fuel in order to produce heat, the thermodynamic limits on efficiency are much higher than for normal power generation processes. In essence, a fuel cell consists of two electrodes separated by an ion-conducting material or membrane. The ion-conducting membrane must allow the diffusion of ions from one electrode to the other and at the same time prevent the flow of electrons across the membrane and keep the fuel and oxidant components apart. If electrons are able to cross the membrane, the fuel cell will be fully or partially shorted out, and any useful power that has been produced will be eliminated or reduced. Diffusion or leakage of the fuel or oxidant across the membrane can also result in undesirable consequences.
Early fuel cells incorporated a liquid electrolyte such as for example, an acid, alkaline or salt solution, as the ion-conducting material. With advances in technology, however, interest has shifted to the development of solid electrolyte ion-conducting membranes, such as the solid proton exchange membrane, DuPont Nafion.RTM.. Solid electrolyte membranes provide several advantages over liquid electrolyte compositions. For example, a fuel cell having a solid electrolyte membrane does not contain any corrosives or solvents that might react with the seals or other portions of the fuel cell container. In addition, with solid electrolyte membranes, fuel cells may be constructed that are thin and lightweight and wherein a plurality of cells may be stacked. Electrolyte compositions have been developed that have good film-forming properties and that therefore can form membranes having good flexibility and mechanical strength and that exhibit high conductivity.
Solid electrolytes can be broadly divided into two groups--organic and inorganic. Organic solid electrolytes, while typically exhibiting lower ionic conductivity, provide good mechanical properties and flexibility and are able to form thin films. Inorganic solid electrolytes on the other hand, while generally having relatively high ionic conductivity, exhibit poor mechanical strength due to their crystalline nature.
Over the past two decades, a wide variety of solid electrolyte compositions have been investigated for use in electrochemical devices such as fuel cells and batteries. In 1973, for example, Dr. P. V. Wright reported a class of solid electrolytes for use in a lithium ion battery. The electrolyte material comprises a polymer such as poly(ethylene oxide), (--CH.sub.2 CH.sub.2 O--).sub.n, or "PEO", and a lithium salt.
Gray et al., "Novel Polymer Electrolytes Based on ABA Block Copolymers," Macromolecules, 21:392-397(1988) discloses a styrene-butadiene-styrene block copolymer wherein the ion-conducting entity is a pendant short-chain PEO monomethyl ether complex with LiCF.sub.3 SO.sub.3 salt which is connected through a succinate linkage to a flexible connecting entity which is the butadiene block of the triblock copolymer.
U.S. Pat. No. 4,828,941 to Stenzel discloses an anion exchanger solid electrolyte polymer-based membrane for use in a methanol/air fuel cell.
U.S. Pat. No. 5,643,490 to Takahashi et al discloses a polymer solid electrolyte composition that is comprised of an organic polymer having an alkyl quaternary ammonium salt structure and a cold-melting salt. The salt component is the reaction product of a nitrogen-containing heterocyclic quaternary ammonium salt and a metal salt, preferably an aluminum halide.
Other polymer-based solid electrolyte materials include composites of PEO and alkali metal salts, such as for example, Na salt; acrylic or methacrylic, organic high polymers having a PEO structure at its side chain; polyphosphazenic, organic polymers having PEO structures as its side chains and (--P.dbd.N--) as its main chain; and siloxanic, organic polymers having a PEO structure at its side chain and (--SiO--) as its main chain. Such polymer-based materials however, while having high ionic conductivity, typically function only at extremely high temperatures (100.degree. C. or higher) and are therefore inappropriate for use in ordinary fuel cells and batteries that are generally used at room temperature. In addition, the flexibility and film-forming properties of these materials are typically less than desirable.
With the recent development of H.sub.2 /O.sub.2 fuel cell technology, attention has been focused on the development of proton transport/exchange membranes. In the early 1970's , for example, for reasons of chemical stability, DuPont introduced a fully fluorinated polymer membrane, Nafion.RTM., which has since served as the basis from which subsequent proton exchange membrane fuel cells have traditionally been constructed. Nafion.RTM. belongs to a wide class of solid superacid catalysts exhibiting acid strength greater than that of 100 percent H.sub.2 SO.sub.4. The composition includes both hydrophobic (--CF.sub.2 --CF.sub.2 --) and hydrophilic (--SO.sub.3 H) regions in its polymer backbone and the strong acidic features of the composition are the result of the electron-withdrawing effect of the perfluorocarbon chain on the sulfonic acid group. Nafion.RTM. however, is very expensive to produce, thus raising the cost of fuel cells to a level that renders them commercially unattractive. As a result, attention has therefore been focused upon the development of a less expensive proton-conducting material.
U.S. Pat. No. 5,468,574 to Ehrenberg et al. discloses a proton-conducting membrane comprised of a plurality of acid-stable polymer molecules each having at least one ion-conducting component covalently bonded to at least one flexible connecting component. The membrane is characterized as a highly sulfonated polymeric membrane composed of block copolymers of sulfonated polystyrene, ethylene and butylene blocks.
In 1997, NASA's Jet Propulsion Laboratory disclosed the development of an improved proton-conducting membrane for use in both H.sub.2 /O.sub.2 and direct methanol fuel cells. The membrane material is composed of highly sulfonated poly(ether ether ketone), commonly known as H-SPEEK. In comparison with previous fuel cell membrane materials, H-SPEEK is claimed to be more stable in the optimum range of operating temperatures (100 to 200.degree. C.), to be less permeable by methanol, and to be much less expensive to produce. See, "Polymeric Electrolyte Membrane Materials for Fuel Cells," NASA Tech Briefs, p. 64, September 1997.
As attention continues to be focused on the development of less expensive proton-conducting fuel cell membranes, the present inventors have discovered the importance of another type of ion-conducting membrane--one that transports hydroxide ion. The transport of hydroxide ion is considered to be the basis for the operation of power sources as alkaline batteries and fuel cells. Accordingly, the present inventors have recognized that in order to apply the many advantages of solid electrolyte membranes to alkaline power sources, it is necessary to provide a hydroxide-conducting composition having good film-forming properties, including flexibility and mechanical strength. A film formed of the material must allow the diffusion of hydroxide anion and a the same time prevent the flow of electrons and the diffusion of molecular gases. Previously known electrolyte compositions such as alkali metal ion-exchange and proton-exchange materials do not satisfy these criteria and therefore, cannot function as a hydroxide conducting solid electrolyte membrane.
Prior to the present invention, aqueous alkaline solutions, such as potassium hydroxide and sodium hydroxide, were utilized as the liquid electrolyte in alkaline batteries and fuel cells. The function of the electrolyte solution is to provide the hydroxide anion responsible for conducting ion transport from one electrode to the other in the operation of the electrochemical cell. Recognizing the value of solid electrolyte membranes, the present inventors have discovered a polymer-based electrolyte composition that may be cast in the form of a film and substituted for the liquid electrolyte solution in an alkaline battery or fuel cell.
In order to function as a solid electrolyte membrane in an alkaline battery or fuel cell, a material should contain high-density hydroxide carrier ions; it should have functional groups capable of adequately interacting with the hydroxide ion carrier ions; it should maintain its amorphous state even at low temperatures (e.g. room temperature); and it should be free of electronic conduction. The polymer-based electrolyte composition of the present invention satisfies each of these requirements.